They do sell stamps, but they are the expensive special addition kind. It would be much cheaper to buy them in advance. Since that might not be possible coming from Canada, maybe try the hotel’s business center or if you will have a rental car, run by a local post office.
The other issue is there is no one there who can tell you how many stamps you’d need to use. If there’s any way you can get a prestamped / prepaid for envelope or card in advance, that is what I’d do. I know in the US you can go to the local post office or UPS and give them the starting and ending zipcodes, and they will figure out the correct postage for you. You can then buy the correct number of stamps.
The owl post is really only a mail drop off spot. There is a TM there, but they aren’t a US postal worker. They are just a regular Uni employee. They have a little owl post stamp they use, but it’s just for fun. It doesn’t really mean anything as far the post office is concerned.
You can also send mail by rocket ship at the front of the park, but again it is just a drop off mailbox. There isn’t even a TM at this spot, so everything needs to be prestamped with the correct postage.